1. Field of the Invention
The present invention relates in general to belt type continuously variable transmissions and more particularly to the belt type continuously variable transmissions of a double piston type wherein two piston chambers are provided for each of primary and secondary pulleys for controlling rotation of the pulleys.
2. Description of the Related Art
In the belt type continuously variable transmissions, an endless belt is put around the primary and secondary pulleys in such a manner that each pulley holds the belt to carry out a thrust transmission therebetween. That is, the thrust transmission is carried out between each pulley and the belt with the aid of a frictional force generated therebetween. However, if the frictional force generated is not sufficiently large as compared with the belt driving force, a belt slippage takes place which induces not only a poor thrust transmission between the pulley and the belt but also a wearing of the belt and/or the pulley.
Accordingly, in the double piston and belt type continuously variable transmissions such as one disclosed in Japanese Laid-open Patent Application (Tokkai) 2002-327814, each of the primary and secondary pulleys is provided with two piston chambers, one being a piston chamber constructed to control the speed of the pulley and the other being a clamp chamber for suppressing the belt slippage. In the published Application, the respective clamp chambers of the primary and secondary pulleys are connected through a common passage for applying these two pulleys with an even clamping pressure. In addition to this, measures are further employed by which input/output flow of the hydraulic pressure into and from the clamp chambers is reduced as little as possible.